Erosion on Earth is the process of wearing away landforms via the persistent action of wind, water, or ice. The Moon, however, lacks this terrestrial process because it is missing the fundamental ingredients that drive weathering and sediment transport: a substantial atmosphere, a circulating water cycle, and an active, dynamic geology. This absence means that features on the lunar surface, like the footprints left by Apollo astronauts, can remain virtually unchanged for millions of years.
Missing Elements: Atmosphere and Water Cycle
The primary factor preventing erosion on the Moon is its near-vacuum environment, technically called an exosphere. This extremely thin layer of gases is insufficient to generate any atmospheric pressure, meaning there is no wind to scour the surface or abrade rock formations. On Earth, wind is a powerful agent of erosion, but the lack of an atmosphere on the Moon eliminates this form of mechanical weathering.
The absence of an atmosphere also means there is no hydrological cycle, eliminating the erosive power of liquid water. There are no oceans, rivers, or rain to dissolve minerals, carve canyons, or transport sediment. Liquid water is the primary driver of chemical and physical erosion on Earth, and its non-existence on the Moon preserves surface features for billions of years.
While trace amounts of water ice exist in permanently shadowed craters, this water is not in a flowing, liquid state that could cause erosion. The lack of an atmospheric blanket also prevents the formation of frost, which eliminates the powerful freeze/thaw cycles that break apart rocks on Earth. Without the moderating effect of a dense atmosphere, the lunar surface is exposed to extreme temperature swings, ranging from approximately 120°C in sunlight to -170°C in shadow.
The Role of Tectonic Inactivity
The Moon’s surface is preserved because it is largely geologically static, lacking the Earth’s engine for surface renewal: plate tectonics. Earth’s crust is constantly recycled by movement driven by internal heat and mantle convection, which builds mountains and volcanoes that are then subject to erosion.
The Moon, being much smaller, cooled far more quickly in its history, leading to a rigid and static crust. This lack of internal heat and convection means the Moon has not experienced the continuous creation of new crust, mountain building, or large-scale volcanism for billions of years.
Consequently, the vast majority of the lunar surface is ancient, with over 70% of it forming more than four billion years ago. On Earth, less than 20% of the surface is that old, demonstrating the difference in geological activity.
Although the Moon is generally considered geologically dead, recent evidence suggests minor tectonic activity is still occurring. Data from the Lunar Reconnaissance Orbiter has revealed thousands of small thrust faults, or scarps, formed as the Moon slightly shrinks and wrinkles due to gradual cooling. This process creates shallow moonquakes and suggests the surface is still evolving, but it does not constitute the widespread tectonic movement seen on Earth.
Surface Alteration Processes on the Moon
While the Moon does not experience terrestrial erosion, its surface is altered by slower processes unique to an airless body. The most dominant process is impact gardening, or micrometeorite bombardment. Since there is no atmosphere to burn up incoming space debris, the surface is constantly struck by high-velocity particles.
These continuous impacts pulverize the surface rock into regolith, an unconsolidated layer of fine dust and fragmented material. This mechanical process slowly churns and mixes the top layers of the lunar soil over vast timescales. Impact gardening does not transport material across long distances like a river, but rather erodes and modifies the surface in situ.
Another process is space weathering, which involves the interaction of the surface with the solar wind and cosmic rays. Solar wind is a stream of charged particles that implants itself into the regolith grains, causing changes in the chemical and optical properties of the lunar soil. This results in the gradual darkening and aging of the surface. This process alters the material itself but does not carve or transport geological features.
Finally, extreme thermal cycling contributes to rock breakdown through thermal fatigue. Rapid temperature shifts between lunar day and night cause surface rock to expand and contract repeatedly. Over millions of years, this mechanical stress can lead to the fracturing and disintegration of rock, contributing to the fine regolith layer.